Method of making variable resistance units



NOV, 10;, 1936 L PODQLSKY METHOD OF MAKING VARIABLE RESISTANCE UNITSFiled April 26, 1934- INVENTOR 6 0 06 ATTORNEYS viv-w 4/ av Arc ofRotczzaan Patented Nov. 10, 1936 UNITED STATES PATENT OFFICE.

METHOD OF MAKING VARIABLE RESISTANCE UNITS Leon Podolsky, Philadelphia,Pa., assignor to Wirt Company, Philadelphia, 2a., a corporation oi MaineMy present invention has for its object to provide a new type ofresistance unit for use in connection with electric circuits which is soconstructed that the voltage or flow of current may be regulated in aplurality of successive steps and at the same time graduated orregulated within each step, the successive steps becoming cumulative intheir action.

A further object of my invention is to provide a current regulatorcomprising a strip member having successive areas of differentresistivities forming a terminal of a circuit in association with acontact adapted to be the other terminal of such circuit, said twomembers being so mounted that relative linear movement may be hadwhereby the contact may traverse all the various sections or areas ofthe strip.

My invention also embraces the method of constructing resistance unitswith electrically connected areas of different resistive values and asimple apparatus for coating either sheets or continuous ribbons ofsuitable material with parallel longitudinal bands of carbonaceousmaterial from which the unit'strips may be formed by severing I thesheets or ribbons transversely into desired widths.

To these and other ends my invention consists in further improvementsall as will be more fully described, the novel features being more fullypointed out in the appended claims.

In the drawing:

Fig. 1 is a diagram illustrating several different resistance curvesshowing the capabilities of resistance units made in accordance with myinvention.

Fig. 2 is a plan view of a strip member of a resistance unit made inaccordance with my invention.

Fig. 3 is a cross section thereof greatly enlarged.

Fig. 4 is a diagrammatic perspective view illustrating one form ofassociating the contact strip with a cooperating circuit terminal.

Fig. 5 is a plan view of a portion of a sheet or ribbon of the coatedmaterial.

Fig. 6 is a diagrammatic view of the principal elements of a coatingmachine for applying the coated areas to sheets such as shown in Fig. 5.

Fig. 7 is a transverse view thereof, taken on the line '|.r'|.r of Fig.6 illustrating the sectional coating rolls, ink tray and smearingidlers.

Fig. 8 is a front view showing one form of a commercial assembly; and

Fig. 9 is a vertical sectional view taken on the line Sx-Sx of Fig. 8. t

The several reference characters appearing on the several figuresindicate similar parts.

In my experiments in various branches of the electrical industry, I havedemonstrated that there are certain types of apparatus which requirevoltage regulation that operate better if, instead of using a step bystep rheostat, or the usual type of continuously regulated voltagedivider, the current is graduated intermediately in progressive steps.Especially is this so in the operation and control of certain types ofthermionic tubes or vacuum tubes employed in the transmission andreception of radio signals.

In Fig. 1, I have shown four different forms of curves indicative of themanner in which the voltage in a given circuit may be controlled to meetany desired requirement by means of a regulator or unit embodying myinvention. Observing curves 0., b, c, it will be noticed that for agiven distance through which the movable contact member first travelsthe resistance increases gradually, that when this reaches a desirednumber of ohms the resistance greatly increases upon additional slightmovement of the contact and rapidly increases as the contact is furtheradvanced until another stage is reached whereupon there is still afurther sudden increase of the resistance which likewise may begradually increased. Curve d is indicative of an action different fromthat just mentioned in which. starting with a high resistance, we haveupon movement of the contact member a lowering of the resistance. Thereis no reasonable limit to the number of control steps which may beprovided but in the great majority of cases three or four such stepswill meet practically all commercial requirements. In the last step themaximum resistance, provided as indicated by curves a, b, and 0, willremain practically constant or rise very slowly upon further movement ofthe contact, or, as shown by curve d will have reached the lowest point.

As shown in Fig. 2 the above objects are accomplished by providing acontact strip having successively disposed areas of conductive materialindicated by I 2 and 3, each of a different resistive value. The lengthof each of said areas is not arbitrarily fixed, as shown, but ispredetermined according to the conditions to be met in the particularcircuit in which the resistance element is employed. Cooperating withthe latter is a longitudinally movable contact 4. The contact may,however, be stationary and provisions made to adjust the resistance, ifdesired. In practice the resistance strip may be conveniently bent intocircular form, as shown in Fig. 4, and

the cooperating contact mounted to rotate therein. In practice theresistance element comprises a lining for the inner periphery of thewall of a suitable container, the wiper or brush 4 being carried upon acentral shaft 5 supported in a suitable hearing which may be rotated bythe knob 6. I

Reliability in an instrument of this kind is of primary importance and Ihave devised a method of making the strip to provide one having a longlife and one that will not vary materially with thermal conditions. Tothis end I provide a base I of a phenol condensation product which inthe process of curing I treat so that the resinous substances are notcompletely hardened. Briefly stated this is accomplished by onlysemicuring the sheet initially, i. e. polymerization is arrested beforeit is completely effected. In the next step I coat the areas, indicatedby i, 2 and 3 in Fig. 2, with resistance materials containing carbon, orsimilar conductive substances, in different proportions.

In carrying out my invention I prefer to make the several resistances influid form and apply them in any approved manner. To this end I may makea primary solution of carbonaceous material, organic resin and anorganic solvent. The exact proportions are not essential but as anexample of a solution which I have satisfactorily used in practice Iemploy 21% of carbon, 26% resin and 53% of solvent. The resin used isone which has an affinity for that in the sheet to which the solution isapplied viz. a phenol-formaldehyde product and the solvent is a selectedone such as acetone or ethyl acetate which will dissolve the resin andalso have a tendency to soften the semicured surface of the sheet. Thecharacter of the carbonaceous material used and the quantity employed,it will be understood, is varied in the several solutions used to coatdifferent areas of the sheet in accordance with the requirements of thedifferent resistances which it is desired to obtain in each of saidareas. After the solution, or solutions are applied to the sheet thecuring of the latter is completed by the application of heat.

In Fig. 6 I have shown diagrammatically the elements of a coatingmachine comprising a coating roller 8 composed of the desired number ofsections rotating in different compartments of a pan I and serving totransfer the several graded solutions of resistance substances to theunder side of a sheet or ribbon ill to coat it with parallellongitudinal strips as shown in Fig. 5. Immediately following thecoating rolls 8 there are small rolls II which overlap the contiguousedges of the band coatings. The lower rolls of each of these pairs ofrolls serve to blend or smear the different coatings together so that inthe finished strips there will be no break in the conductivity from oneend of the strip to the other. The coated sheet finally passes between apair of calender rolls i2 heated to approximately 350 Fahrenheit forcompleting the curing of the,

coated phenol composition sheet and making a perfect union between itand the coating or coatings on its surface. In this way I find I am ableto provide a resistance surface which is bonded to the support orbacking and will wear practically indefinitely.

The method of manufacture described has cer tain other advantages inthat the finished surface is hard, does not rub off and possesses longlife. In some instances I recommend that the coated surface becalendered, in addition to the calendaring action of the rolls II, toburnish it,

as I find that in so doing all sound, in highly amplified circuits, dueto the adjustment of the contact, can be eliminated.

In Figs. 2 and 3 the various areas referred to are shown separated byfaint lines and the different areas are indicated by various degrees ofshading, or stippling. These are merely means I have employed to conveyan idea of the manner in which the invention is carried out as in thefinished product these lines of demarcation are not visible to the nakedeye.

As an example of a satisfactory commercial instrument I have shown inFigs. 8 and 9 a casing I5, preferably of moulded insulating material,having a cylindrical inner wall against which the resistance strip,indicated by ll, rests. Across this surface a small rib I1 extends, theopposite edges of which form abutments for the ends of the strip. Thisfacilitates the assembly and prevents the strip from shifting. Also theadded material at this point of the casing provides space for anaperture is by means of which the casing may be mounted by a suitablebolt to a panel. For electrically connecting the conducting portions ofthe strip with a circuit I provide the terminals II and in the form ofbolts projecting through the wall I! and beneath the heads of the boltssmall contact plates Isa, 20a rest upon the ends of the coated areas ofthe strip IS.

The contact member in this instance, corresponding with the wiper l ofFig. 4, is a roller II moved by oscillating the shaft 22. The latter isjournalled in a sleeve 23 secured to the base of the casing l5 and atits inner end carries a yoke 24. A spring washer 25 lies between theyoke and the inner end of the sleeve to prevent the movement of theshaft in one direction and hold the collar 28 on the shaft in contactwith the outer end of thesleeve. The extremities of the yoke 24 extendoutwardly and are slotted to form a guide for the fiat stem 21 of theroller carrying member. The roller 2| is slotted to receive the forkedouter end 21a of the stem 21. It will be observed that in constructingthismember I carry the enlargement of the end of the stem carrying theroller through the contiguous guide portion of the yoke so as to formshoulders against which rests a washer 28, pressed outwardly by a coilspring 28. In this way the contact roller II is held against theconducting surface of the contact strip with a uniform pressure in allpositions of adjustment notwithstanding slight irregularities which mayoccur.

In the description herein I have referred to a resistance strip havingtwo or more coatings but it will be understood that my invention alsocomprehends a single coating of the nature described which will havemany uses in the electrical art where a continuously progressiveresistance, acting the same as the present wire wound resistors orvoltage dividers, may be used. It will also be appreciated that thedifferent areas indicated, for instance in Fig. 2, may be made in theform of bars of material of different resistance values united end toend to provide a continuous electrlcal path without a backing or supportand may be made either fiat or curved.

I claim as my invention:

1. The method 'of making resistor elements comprising applying to thesurface of an incompletely polymerized sheet of phenol condensationproduct a solution containing carbonaceous material aphenol-formaldehyde resin and a solvent for said resin and sheet andsubsequently completing the polymerization of said sheet with thecoating on the surface thereof.

2. The method of making resistances consisting of the steps ofsemicuring a non-moisture absorbing base composed of a sheet of phenolcondensation product, coating it on the surface with adjacent strips ofconductive material possessing phenol-formaldehyde having an afiinityfor the substances of which the base is composed and completing thecuring of the latter leaving the conductive material on the surface ofthe base.

3. The method of making resistances comprising applying thin coatings ona sheet of semicured phenol condensation product as adjacent hands ofconductive material of different resistance values, uniting the adjacentedges of said bands, heating the coated strip to complete the curingthereof and cutting the sheet transversely of said bands to form aplurality of strips.

4. The method of making resistances comprising coating one surface of asheet of partially cured phenol condensation product with adjacent bandsof conductive material of diiferent resistance values containingphenol-formaldehyde substances having an afllnity for the materialscomposing said sheet and completing the curing thereof to effect a bondbetween the sheet and the coatings thereon.

mos PODOLSKY. w

